Golf ball covers can be made by injection molding thermoplastic cover material about a core. It is standard practice in injection molding to provide a mold having two cavities, each having hemispherical surfaces that mate when the mold is closed. At the initial stage of the covering process, the core of the golf ball is supported centrally within the cavities by retractable pins or the like so as to leave a space for molding the cover about the core. A thermoplastic cover material is then injected into the mold cavity through a plurality of gates. Flow of the cover material from each of the plurality of gates eventually joins to fill the void between the golf ball core and the mold. Once the void is nearly filled, but before the cover material has completely hardened, the pins holding the core retract so that the cover material may fill the voids left by the pins. The cover material then cools and hardens to form the cover.
Referring to FIG. 1, golf ball 10 includes a spherical outer surface 12 with a plurality of dimples 14 formed thereon. Dimples 14 are circular depressions that influence the ball's flight Golf ball 10 also includes knit lines 16. Knit lines 16 are formed where the flow fronts of cover material from separate gates meet. This meeting point can be referred to as a flow terminus. The knit lines are areas where the cover material's tensile strength maybe substantially reduced, and knit lines usually are masked by finishing processes. As a result of the decreased strength and cosmetic appearance, knit lines are undesirable.
In an effort to reduce the occurrence of knit lines, manufacturers have prepared molds in various ways. For example, one industry standard for making retractable pin injection molds includes first producing a hob that is formed in a lathe with a single point tool. The tool leaves radial marks or grooves on the surface of the hob. The surface of the hob is usually ground or polished to reduce the appearance of the tool marks. The hob is forced into a piece of stock to form the mold cavity and the resulting mold cavity is ground and/or polished to have a smooth appearance with a low average surface roughness of less than 10 microinches. Golf balls produced by such molds, however, have an undesirably high occurrence of knit lines.
U.S. Pat. Nos. 5,827,567 and 6,003,724 disclose other golf or game ball mold preparation techniques. These techniques include contacting a mold cavity with an abrasive media until substantially all of any previous coating has been removed and blasting the mold with aluminum oxide powder. These techniques prepare the mold to accept a mold release coating, which is subsequently applied to the mold cavity to fill any voids created by the blasting. These techniques have several drawbacks. First, masking may be necessary to keep coating off particular areas of the mold, such as pin holes. Masking is labor intensive. In addition, adding coating adds costs and the coating must be periodically checked and reapplied to be effective.
Injection molds may also have venting problems. Usually, such molds include a vent located at or near the intersection 18 of the knit lines 16, as shown in FIG. 1, so that trapped air and gasses at the flow terminus can be released through the vent. When the flow from the gates is unbalanced, the flow terminus is not near the vent, and the trapped air and gasses cannot evacuate the cavity effectively. This can cause voids in the cover in one location and cause excess material in another location, which can decrease the cover strength and/or increase the need for post-mold finishing.
One way to address unbalanced fill caused by the location and geometry of the gates is to modify the size of each gate manually using a file or the like. This solution, however, is labor intensive and its proper execution requires a great deal of skill and experience. Furthermore, such a balancing increases in difficulty as the number of cavities in the mold increase. When the mold is used with different products whose cavity size and injection moldable material may vary from that used during manual adjustment, placing new cavities in the mold may require additional adjustments, which is undesirable. In order to periodically service the cavities, they are typically removed from the mold. If the cavities are removed after manual adjustment, they may have to be returned to the same mold and same exact location in the mold to maintain the balance. This can be difficult to accomplish from a tool management perspective.
Therefore, there exists a need for a method of preparing golf ball molds for injecting molding which reduces or substantially eliminates the occurrence of knit lines, thereby increasing the durability of the golf ball cover and extending the useful life of the golf ball, without requiring manual adjustment of the gates in the mold and without significantly effecting dimples geometry or the ball finish.